Wires are formed on a circuit board in order to transmit and receive electric signals to and from ICs (Integrated Circuits), semiconductor components, and other electronic components mounted on the circuit board. With recent miniaturization of electronic components, wires which are connected to the electronic components have been formed more miniaturized and complicated. Moreover, these wires have been formed to have lower resistance than ever before.
Higher accuracy methods are required to conduct wire defect tests on such wires. Due to the resistance of the miniaturized wire, slight error or poor accuracy causes an incorrect result of a test as to whether or not a resistance value of each wire is acceptable.
Particularly, signal wires are formed finer on a circuit board so as to be of low resistance. Therefore, a two-terminal measurement method fails to calculate an accurate resistance value of each signal wire because of a significant influence of contact resistance. In order to overcome such a disadvantage, a four-terminal measurement method has been employed, which does not undergo the influence of contact resistance.
The four-terminal measurement method does not take the contact resistance into consideration. In conducting a test, therefore, a terminal (contact probe) for power supply and a terminal for detection and measurement are brought into contact with each test point defined on a wire to be subjected to the test.
As wires are formed finer on a circuit board, a testing jig for use in the four-terminal measurement method has a plurality of contact probes disposed with narrower pitches. In the four-terminal measurement method, particularly, a contact probe for power supply and a contact probe for detection and measurement (a pair of contact probes) are disposed with considerably narrower pitches.
In performing the four-terminal measurement method, as described above, two contact probes are brought into conductive contact with one test point. Prior to the measurement of a resistance value between test points in the four-terminal measurement method, therefore, it is determined whether or not two contact probes are in conductive contact with each test point. In order to determine whether or not two contact probes are in conductive contact with a test point set in advance, the four-terminal measurement method involves applying a current to one of the contact probes and detecting the current flowing into the other contact probe via the test point. If the current from the other contact probe cannot be detected, it is determined that one of the two contact probes is not in conductive contact with the test point. In order to achieve a favorable conductive contact state, a testing jig is spaced apart from the circuit board, and then is brought into contact with the circuit board again.
According to the four-terminal measurement method, conductive contact states of two contact probes with a test point are confirmed for each test point. The four-terminal measurement method involves performing a measurement for confirming a conductive contact state of a contact probe with a test point defined on one end of a wire, performing a measurement for confirming a conductive contact state of a contact probe with a test point defined on the other end of the wire, and measuring a resistance value between the test points. Therefore, the four-terminal measurement method requires measurements which are three times as large as that of the ordinary two-terminal measurement method. Hence, an increase in time for the test causes a reduction in productivity.
JP 2013-024724 A discloses a method for confirming conductive contact states of two contact probes with a test point (i.e., performing a contact check) in a four-terminal measurement method. According to the method in JP 2013-024724 A, in order to reduce time for the contact check, conductive contact states of two contact probes with a test point on one end of a wire to be subjected to a test are confirmed in a state in which the contact probes are connected in series.
However, the method in JP 2013-024724 A involves performing a test measurement with contact probes, which are in conductive contact with a test point on one end of a wire to be subjected to a test, connected in series, and performing a test measurement with contact probes, which are in conductive contact with a test point on the other end of the wire, connected in series. That is, at least two test measurements are performed for the contact check.
According to the method in JP 2013-024724 A, moreover, if the two contact probes are short-circuited, a closed circuit is formed although the two contact probes are not in conductive contact with the test point. As a result, it is erroneously determined that the contact probes are in conductive contact with the test point.